Added Lua module rtctime to interface with it.
This keeps as accurate time as is possible on the ESP8266, including across
deep sleeps (provided rtctime.dsleep() is used rather than node.dsleep()).
Intended to be used together with NTP for high accuracy time keeping. The
API is via rtctime.{get,set}timeofday(), working from Unix epoch.
Note that 160MHz CPU clock is not currently supported by the rtctime code,
as it is only aware of the 52MHz boot clock and the regular 80Mhz default
clock.
See rtctime.h for detailed info on how this all works.
